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The strongest earthquakes that strike the planet, such as the
9.0-magnitude earthquake that hit Japan last year, occur at
particular "hotspot" points of Earth's crust, a new study finds.

About 87 percent of the 15 largest earthquakes in the last
century occurred in the intersection between specific areas on
spreading ocean plates, called oceanic fracture zones, and
subduction zones, where one tectonic plate slides underneath
another, according to the paper, published recently in the
journal Solid Earth. The scientists used a data mining method to
find correlations between locations of earthquakes over the last
100 years, the strength and geological origin.

The bottom of the ocean is crossed by underwater ridges, such as
the
mid-Atlantic ridge, which runs north to south between the
Americas and Africa. These ridges divide two tectonic plates that
move apart as lava emerges, solidifying and creating new rock.
The midocean ridge jogs back and forth at offsets known as
transform faults, creating zigzag-shaped plate boundaries.
Fracture zones are scars in the ocean floor left by these
transform faults.

These fracture zones are often marked by large underwater
mountains with valleys between them. Millions of years after
forming in the middle of the ocean, these mountains slowly
advance all the way to a subduction zone, often at the opposite
end of the sea. The researchers hypothesize that these
submarine mountains "snag" as they enter subduction zones,
causing an enormous amount of pressure to build up over hundreds
or thousands of years before finally releasing and creating huge
earthquakes, according to the study.

These areas — where the mountains of fracture zones are forced
beneath another plate — are prone to earthquake "supercycles,"
where large earthquakes happen every few hundred or few
thousand years, said Dietmar Müller, a study author and
researcher at the University of Sydney, in a statement.

Many of these areas may not be known to be particularly risky,
since seismic hazard maps are constructed mainly using data
collected after 1900, he said. For example, the area that spawned
Japan's deadly 9.0-magnitude Tohoku quake in 2011 was not
predicted to be of significant risk by previous hazard maps,
according to the study.

"The power of our new method is that it does pick up many of
these regions and, hence, could contribute to much-needed
improvements of long-term seismic hazard maps," Müller said.

While 50 of the largest earthquakes in the past 100 years have
also happened in the fraught regions between these fracture zones
and subduction zones, the connection doesn't seem to hold for
smaller quakes, according to the study. That's because other
faults aren't "blocked" in the same way by large underwater
features, and don't need to accumulate as much stress before
faulting, the researchers say.

The paper hasn't officially been peer-reviewed, although many
scientists have
commented on the study online. "I ﬁnd the evidence of
positive correlation presented in this paper convincing enough,"
wrote one scientist. "Given the small amount of data available,
the authors have developed an interesting way of testing for
correlations."